I'm no genius but from what I understand rust eats away at metal. If the paint chips and the frame is exposed, then that area will rust. Possibly the rust won't spread under the paint but it will eat the metal and produces holes. Those holes will then expose more metal and from there on it's a domino effect. I haven't had any experience with POR-15 but I have read up on it. My fiancee and I are going to start playing with our frame so I am following this thread with great interest. Take care everyone. Marni VIN 7104